This invention relates to a method of treating the flue dust of industrial processes, such as the flue dust from steel making processes which contain such materials as zinc, lead or cadmium, to segregate and thereby recover valuable constituents.
Flue dust from certain steel making operations such as electric arc furnaces is classified as a hazardous waste material because of the presence of lead and cadmium. Also, it has been reported that electric arc furnace dust xe2x80x9ccontains several thousand nanogrammes of dioxins per kilogramme of dustxe2x80x9d. xe2x80x9cValue from EAF Dustxe2x80x94The ADPL Processxe2x80x9d, by Kevin Holliday, published Apr. 10, 2000. Yet electric arc furnace dust contains valuable components if separation of the various constituents can be made economically. A process for recovering steel mill and foundry dusts containing zinc is disclosed in U.S. Pat. No. 3,850,613, which is herein incorporated by reference in its entirety. The flue dust is formed into compacts and then heated to reduce the zinc and lead to a metal, and then to volatilize the metal. The metal vapor is then combined with oxygen to form gas-borne metallic oxides.
A commonly used process for the processing of flue dust is the Waelz process, as disclosed in U.S. Pat. No. 4,525,208, which is also herein incorporated by reference in its entirety. The Waelz kiln process involves mixing carbon, usually in the form of coke or coal, with the flue dust to form a mixture, and then forming the carbon/flue dust mixture into compacts. The compacts are heated in a reducing atmosphere to volatilize the metallic zinc, and the metallic zinc is later reoxidized to form zinc oxide, a stable material having a high melting temperature. The process is carried out in a counterflow type rotary kiln. The iron is discharged in the form of reduced compacts, and the zinc oxide is carried gas-borne along with the kiln exhaust stream. A substantial portion of the heat, if not all of the heat required to initially vaporize the zinc is generated by the burning of the carbon in the carbon/flue dust mixture within the kiln. The combustion process within the kiln requires a substantial amount of air/oxygen, and the flow of this air, and the combustion process itself, generate a significant amount of turbulence within the kiln. As a result of this turbulence, a substantial amount of iron, calcium, silicon and aluminum compounds are also made gas-borne within the kiln, and these contaminate the exhaust stream of potentially valuable zinc oxides.
U.S. Pat. No. 5,013,532, discloses a process using a stream of hydrogen gas to reduce the zinc contained in electric arc furnace dust, followed by volatilizing the metallic zinc, and reoxidizing the metallic zinc to form zinc oxide. A process for removing zinc and other heavy metals from compacted furnace dust and carbon mixture is disclosed in U.S. Pat. No. 5,186,741, which is herein incorporated by reference in its entirety. A process for removing zinc, lead and cadmium from electric arc furnace dust, and metallizing the iron present in the dust to return it to the steel making operation is disclosed in U.S. Pat. No. 5,601,631, which is herein incorporated by reference in its entirety.
In addition to the processes for handling flue dust disclosed above, it is known to process flue dust in a rotary kiln that is heated exclusively by a heat source external of the kiln, such as by gas combustion on the outside of the kiln. The carbon necessary for the endothermic chemical reaction (to reduce the zinc and cadmium to a metallic, vaporized form) is supplied from the coke mixed with the flue dust. However, the lack of oxygen within the kiln substantially prevents combustion of the carbon, and essentially no heat is supplied to the flue dust internally of the kiln. Such an indirectly heated rotary kiln process is disclosed in a paper entitled xe2x80x9cFirst Experiences and Results of the BSN-Process to Recover Zinc and Lead from EAF Dustxe2x80x9d, by Karl Haase, published May 17, 1999.
It would be advantageous if there could be developed flue dust handling processes that are even more efficient than those described above. Preferably, such a method would reduce the energy consumed in the process, and would result in greater purity of the separated constituents in the various output streams. Also, ideally, the ability to treat environmentally unfriendly substances such as dioxins and furans would be enhanced.
The above objects as well as other objects not specifically enumerated are achieved by a method of processing flue dust, where the flue dust contains one or more compounds from a first group of zinc, lead and cadmium compounds, and contains one or more compounds from a second group of iron, silicon, calcium, magnesium and aluminum compounds. The method includes mixing the flue dust with a carbonaceous material, heating the flue dust/carbonaceous material mixture under non-turbulent conditions to cause a substantial portion of the compounds from the first group to become gas-borne while retaining a substantial portion of the compounds of the second group in a non-gas-borne condition, and separating the gas-borne compounds from the non-gas-borne compounds.
According to this invention, there is also provided a method of processing flue dust, where the flue dust contains one or more compounds from a first group of zinc, lead and cadmium compounds, and contains one or more compounds from a second group of iron, silicon, calcium, magnesium and aluminum compounds. The method includes mixing the flue dust with a carbonaceous material, introducing the mixture of flue dust and carbonaceous material into a heating vessel and heating the vessel from a primary heating source. The carbonaceous material of the mixture within the vessel is combusted to further heat the flue dust. The heat from the primary source and the heat from the combustion of the carbonaceous material cause a substantial portion of the compounds from the first group to become gas-borne while retaining a substantial portion of the compounds of the second group in a non-gas-borne condition. The gas-borne compounds are separated from the non-gas-borne compounds.
According to this invention, there is also provided a method of processing flue dust comprising heating the flue dust to produce a gas stream containing one or more compounds from a first group of zinc, lead and cadmium compounds, where the gas stream further contains one or more compounds from an additional group of metallic chlorides and alkali metal oxides. The gas stream is maintained at a temperature at which the compounds from the additional group are gaseous and the compounds from the first group are solid, and the compounds from the additional group are separated from those compounds from the first group.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.